The objective of this research is the elucidation of the molecular events in tumor cells of pituitary and nonpituitary origin by which the synthesis, storage and secretion of certain polypeptide hormones are regulated. In particular, it is concerned with the biosynthetically related series of hormones derived from the common precursor, pro-opiolipomelanocorotin (proOLMC): adrenocorticotropin (ACTH), beta-lipoprotein (beta LPH), gamma LPH, beta-melanocyte-stimulating hormone (beta MSH) and beta-endorphin (beta END). We prepared poly(A)-RNA from a thymic carcinoid tumor metastatic to the pancreas that had caused the ectopic ACTH syndrome. Complementary DNA (cDNA) was inserted into the Pst I site of plasmid pBR322, and a recombinant bacterial plasmid, pMS1, was constructed that contains 318 nucleotides complementary to a portion of the proOLMC mRNA, including all of the beta MSH and beta END portions of proOLMC, as well as the 3' nontranslated sequence. The sequence is identical to the published sequences for human proOLMC genomic DNA from placenta and fetal liver. Thus, the nucleotide sequence of this portion of the mRNA and the ectopic hormones whose synthesis it directs are identical to the eutopic mRNA and peptide molecules, suggesting that the ectopic and eutopic hormones represent expression of the same gene. However, two poly(A)-RNA hybridizing species of about 1150 and 1300 bases were identified in the tumor, but only one species was observed in pituitary poly(A)-RNA. The additional tumor mRNA may be the product of a different proOLMC gene, or it may be a different product of the same gene because of incomplete or altered processing by the tumor. These results, which represent the first direct sequence of an ectopic hormonal mRNA and the first indirect sequence of an ectopic hormone, have furthered the ultimate aim of this research, which is to understand the relationship between abnormal hormone synthesis and the mechanism of malignant transformation.